Light source



March 7, 1961 w. A. MARRISON LIGHT SOURCE 2 Sheets-Sheet 1 Filed Nov. 12, 1959 WARREN A.MARRISON IN VEN TOR.

2 T G N F M Y B N L 4 \N\ 8 0/ 6 6 0 8 6 I/ 7 II 0 6 7-3 R 8 l 6 I O I 4 v V W Wu C m M m 4 A l 1 2 C I HH/ 2 6 2 4/ 2 7 7 ll v4 2 R v m 6 w 8 Y m 4 N M I j m m m F 2\/\\ l 8 & E i

A TTORNEY March 7, 1961 w. A. MARRISON 2,974,243

LIGHT SOURCE Filed Nov. 12, 1959 2 Sheets-Sheet 2 lI- W WARREN A. MA'RRISON IN VEN TOR.

A TTOR/VE) United States Patent LIGHT SOURCE Warren A. Marrison, Palos Verdes Estates, Calif., assignor to Space Technology Laboratories, Inc., a corporation of Delaware Filed Nov. '12, 1959, Ser. No. 852,266

19 Claims. (Cl. 313-46) This invention relates to light sources employing vapor discharge lamps and more particularly to improvements in vapor discharge lamps of the kind that are devoid of internal electrodes and which receive energy of excitation from electromagnetic fields applied from sources that are external of the lamp.

Electrodeless vapor discharge lamps are known in which light emission is produced through the ionizing action of electromagnetic fields on a vaporizable light radiating substance, such as a vapor confined in an envelope, without the aid of electrodes in the envelope. Such a lamp is comparatively simple in structure, is inexpensive to build and operate, and generally has a long life because of the absence of electrodes. However, such lamps have not proved entirely satisfactory for some applications which require a high degree of freedom from noise, because of the tendency of these lamps to introduce noise through fluctuations in light intensity. One such application, atomic clocks, involves the use of an alkali vapor lamp in frequency standard work utilizing atomic resonance phenomena. Such a lamp is used in conjunction with a gas cell of the same alkali vapor contained in the lamp to effect highly accurate frequency control of a radio-frequency signal; the control is effected by detecting an error signal due to variation in signal frequency, and utilizing the error signal to correct the frequency of the radio-frequency signal. In such frequency control work, any extraneous noise makes it diflicult to detect the error signal.

In such lamps vapor particles apparently agglomerate on the walls defining the lamp envelope. Since the envelope walls are made of an insulating material, the agglomerated vapor particles may become electrically charged through the action of the ionized gas on the particles. It is believed that the sharing of the electric charges by the agglomerated particles, when they coalesce, creates electric field distrubances which react on the vapor to cause spurious light emissions or fluctuations in thet light intensity of the vapor discharge. These spurious emissions or light intensity fluctuations'are a source of noise that interferes wth the proper operation of frequency control apparatus utilizing atomic resonance phenomena.

Furthermore, such lamps generally are not as eflicient as might be desired because of the masking effect on the emitted light by the electric and magnetic field exciting sources.

Accordingly, an object of this invention is to reduce noise in a vapor discharge device of the kind that is free from electrodes. V

A further object is to provide novel and improved arrangemen ts for applying electromagnetic energizing fields to a vapor discharge lamp of the kind described above without impairing the light output efliciency.

The foregoing and other objects are realized in a lamp structure that employs a vapor condensing member mounted within the envelope containing the vaporizable ductive material that is easily wettable by the vaporizable substance. The Vapor condensing member is mounted in thermal exchange relationship with a heat dissipating means or heat sink that is disposed externally of the envelope. The heat sink is made of a material having a high thermal conductivity and high heat radiating properties so as to maintain the condensing member cooler than the envelope walls during operation of the lamp. Because of the lower temperature of the condensing mem ber relative to the envelope walls, any excess amount of vapor. of the light radiating substance that is present during lamp operation is deposited on the condensing member rather than on the envelope walls. This results in a substantial reduction in lamp noise.

According to another feature of the invention, electromagnetic field producing elements are positioned to confine light emission from the lamp to substantially only the region of the lamp where no obstruction of the light can occur.

In the 2 sheets of drawing, wherein like reference characters designate like parts:

Fig. 1 is a view partly in section, showing a light source constructed in accordance with the invention, and partly schematic, showing circuit means for supplying electrical power to the light source;

Fig. 2 is a partial sectional view of a modified form of light source embodying the invention;

' Fig. 3 is partly sectional, partly schematic View showing a further modified form of a light source of the invention;

Fig. 4 is an elevation view, partly in section, showing a cooling means useful in the light sources of Figs. 1 to 3;

Fig. 5 is a sectional view taken along line 55 of Fig. 4; and

Fig. 6 is an elevation view, partly in section, showing a further modified form of light source according to the invention.

Fig. 1 illustrates one form of the invention as embodied in a light source intended for use with frequency control apparatus utilizing atomic resonance phenomena. However, it will become apparent that the principles of the invention are applicable to light sources useful in other environments, such as, for example, in the illumination of airport runways. The light source 10 includes a vapor discharge lamp 12 and a reflector 14 mounted within a housing 16. The interior of the housing 16 is provided with an annular shoulder 18 approximately at a central portion thereof for supporting a ring-like mounting bracket 20. The bracket 20 has a large central opening 22, in which the reflector 14 is mounted, and several smaller openings 24 to reduce the cross section of the bracket 20 and thus to thermally insulate the reflector 14 from the housing 16.

The reflector 14, which is made of an electrically conductive material, is generally funnel shaped, there being a neck portion 26 within which the lamp 112 is supported,

The forward or flared end of the reflector 14 is formed 7 with an annular shoulder 30 for supporting a quartz window 31 that is transparent to the light emitted by the lamp 12. The window 31 serves in part as a dust cover for preventing foreign particles from depositing on the inner surface 29 of the reflector 14. Also, since quartz is a good reflector of long wavelength radiation, the window 31 serves to maintain a uniform temperature within the housing 16 by preventing the transmission of heat through the window 31. Both surfaces of the window 31 are preferably provided with anti-reflection coatings 32 and 34, for example of magnesium fluoride, so as to insure maximum transmission of the desired wavelength of light emitted from the lamp 12. The window 31 and reflector 14 are fixed in position by means of an annular washer 36 of heat insulating material and a retaining ring 38 which is screwed into-the forward end of the housing 16.

The discharge lamp 12 has an elongated cylindrical transparent envelope 40 made of glass, for example, one end portion of which is rigidly attached to the neck portion 26 of the reflector 14 by means of an intermediate cement layer 42, such as an epoxy resin, for example. The envelope -40 protrudes from the neck portion 26 into the cavity formed by the flared portion 28 of the reflector 14.

The lamp 12 contains a quantity of ionizable gas, which may be one of the noble gases such as argon, neon, helium, or krypton. In addition, the lamp contains a quantity of a vaporizable substance 44, preferably one of the alkali metals, such as rubidium, caesium, potassium, sodium, or lithium, which, in accordance with the invention is stored on a metallic condensing member 46. The condensing member 46, which may comprise a thin rod of substantially smaller diameter than that of the cylindrical envelope 40, is sealed through one end of the envelope 40 in axial alignment with the envelope 40. When made in rod form, the condensing member 46 may have a diameter that is to the size of the outside diameter of the envelope 40. The condensing member 46 is formed of a material that is easily wettable by the vaporizable substance 44. In addition, the condensing member material should be an electrical conductor. When a vaporizable substance 44 such as rubidium is used, the condensing member 46 may be made of mag sten, for example.

The condensing member 46 is surrounded by a heater 48 which is mounted on the neck portion 26 of the reflector 14. The heater 48 serves to maintain the vapor pressure of the alkali metal vapor at the desired level at which light emission can occur when an energizing field is applied to the gas and vapor. The heater 48 may comprise a helical coil of insulation coated high resistance wire wound around the neck portion 26. A layer 50 of heat insulation material covers the heater 48. The heater 48 may be connected to a source of direct current voltage, not shown, to receive its heating current.

For energizing the lamp 12 a first electromagnetic field producing element in the form of magnetic induction coil 56 is wound preferably around the end of the envelope 40 opposite the end through which the condensing element 46 is sealed. One end of the coil 56 is spaced from the end of the condensing member 46 along the length of the envelope 40 so as to leave an intermediate envelope portion 58 that is free of both internal lamp structure as well as external lamp structure. Furthermore, the lamp envelope 40 is positioned axially within the reflector 14 so that a substantial part of the intermediate unobstructed envelope portion 58 will be centered at the focal point of the reflector 14. Thus, during operation of the lamp 12, a substantial portion of high intensity light emission will issue from the lamp 12 at the focal point of the reflector 14 and emanate from the reflector 14 as parallel light rays. One connection to the induction coil 56 is made through a terminal 60 fastened and conductively connected to the flared portion 28 of the reflector 14. The other connection to the coil 56 is made 4 through apertures 62 and 64 in the reflector 14 and housing 16, respectively.

As discussed previously, one of the problems encountered in atomic resonance apparatus utilizing a prior art vapor discharge lamp of the general type described herein has to do with the suppression of noise which may originate from light intensity fluctuations. The noise encountered in prior art lamps is believed to be associated with the deposition of excess vapor droplets of the alkali metal on the envelope inner wall surfaces. In accordance with the invention, the alkali metal vapor is prevented from depositing on the envelope 40 wall surfaces by the introduction of the condensing member 46. The condensing member 46 is made a preferential collector surface for excess vapor droplets by having its temperature maintained lower than that of the envelope 40 walls. The temperature of the condensing member 46 is lowered by means of a heat sink 66, such as a metal wire, ribbon, or disc, connected to the condensing member 46 and extending externally of the envelope '40. The heat sink 66 is preferably made of high thermal conductivity metal, such as copper, so as to quickly conduct away the heat developed in the condensing member 46 and radiate it to the external surroundings. If it is desired that the heat sink 66 have a greater heat radiating capacity than is furnished by the wire or ribbon alone, the heat sink 66 may be provided with heat radiating fins 68, as shown in Fig. 2.

In one operative embodiment, the envelope 40 was made of Pyrex glass tubing, 7 millimeters in diameter, 1% inches in length. A tungsten rod, .040 inch in diameter, was cleaned electrolytically and sealed in one end of the Pyrex tube so as to project about Ai-inch within the tube and to serve as the condensing member 46. The envelope 40 was evacuated and then provided with an amount of rubidium sufiicient to wet the tungsten rod condensing member. Thereafter, the envelope was filled with argon at a pressure between 3 and 5 millimeters of mercury. Thus constructed, the lamp 12 was mounted inside an aluminum reflector 14.

The energizing field for the lamp 12 was provided by connecting the induction coil 56 in a modified Hartley oscillator circuit, as shown in Fig. l. The induction coil 56, comprising 11 turns of No. 20 copper wire wound around the end of the envelope '40, was connected in series with a second coil 70 physically separated from the induction coil 56. The second coil 70, which was of the same diameter as the inductive coil 56, comprised 8 turns of No. 20 wire. The two series coils 56 and 70 were connected in parallel with a micromicrofarad capacitor 72 to form a tuned circuit. One end of the induction coil 56 was connected to the plate of a type 5763 pentode tube 74 through a .001 microfarad capacitor 76. One end of the second coil 70 was connected to the control grid of the tube 74 through a .005 microfarad blocking capacitor 78. The junction of the two coils 56 and 70 was connected to the cathode of the tube 74 and to the reflector 14 through a common ground connection. The other elements of the Hartley oscillator circuit shown on Fig. 1 are well known filter elements for connecting prescribed unidirectional voltage to various tube elements.

In operation, when the oscillator circuit is energized the alternating magnetic field produced axially of the induction coil 56 induces a circumferential electrical field at right angles to the magnetic field. The circumferential electric field induces ionization of the gas and vapor within the lamp 12 to cause light emission. In addition, there exists an alternating electric field between the high voltage end of the induction coil 56 (opposite the grounded end) and the reflector 14 (which is grounded). The reflector 14 thus constitutes a second electromagnetic field producing element, the first element being constituted by the induction coil 56. Since the composite electric field is concentrated in the intermediate portion 58 of the lamp 12 between the condensing member 46 and the induction coil 56, there will be a concentration of ionization produced in this region. Accordingly, light of relatively high intensity is emitted from the focal point of the reflector 14. It is found that the highest concentration of light emission at the focal point of the reflector 14 results from placing the end of the condensing member 46 at the opening into the flared portion 28 of the reflector 14, as shown, so that the condensing member is substantially coextensive with the neck portion 26. The position of the condensing member 46 appears to be related to a noticeable migration of the vaporizable substance 44 at the extreme tip or hottest part of the condensing member 46, as evidenced by a concentration of the substance 44 at the tip. It is believed that atoms of the vaporizable substance 44 are emitted from the tip of the member 46 and stream toward the focal point of the reflector 14 where they encounter the highest concentration of field.

A preferred arrangement of the condensing member 46 and induction coil 56 is one in which the member 46 and coil 56 are mounted on the envelope 40 opposite each other, with the member 46 and heat sink 66 disposed outside of the flared-portion 28 as shown, so as to provide better heat dissipation. The spaced-apart mounting also insures that the condensing member 46 will not be unduly heated by induction from the induction coil 56. Alternatively, as will be shown, the induction coil 56 may be mounted around the condensing member 46. In such a case, the small diameter of the condensing member 46, relative to the outside diameter of the envelope 40 and thus the inside diameter of the induction coil 56, will prevent overheating of the member 46 by induction currents.

When rubidium is used as the vaporizable material the useful wavelengths of radiation, for atomic frequency standard applications, lie in the region of 7800 angstrom units. In such cases, it is preferred to apply a plating 79 of gold to the inner surface of the reflector 16. Such a plating will increase the reflectivity with respect to light of 7800 angstrom units wavelength, and reduce the reflectivity with respect to the undesired wavelengths (those wavelengths shorter than 7800 angstrom units).

An'alternative light source arrangement is shown in Fig. 3. In this embodiment, the lamp 12 is mounted within a reflector 80 in which the supporting neck portion 26 (Fig. l) is omitted. Instead, the lamp 12 is supported by the housing 16. In more detail, the envelope 40 of the lamp 12 is sealed or otherwise joined to a length of solid glass rod 82. The rod 82 in turn is supported within a cylindrical support member 84 projectlng inwardly from the rear wall 86 of the housing 16, with the rod 82 being fixed within the support member 84 by means of a cement layer 88. The condensing member 46 in this case is disposed within the reflector 80 cavity and is encircled by an induction coil 90. The heat sink 66 may be considerably reduced in length by providing it with the cooling fins 68.

In this embodiment, two similar induction coils 90 and 92 are substituted for the coils 56 and 70 in the circuit shown in Fig. 1. Both coils 90 and 92 are mounted on the lamp 12 and are arranged and connected such that the two fields of the coils are in series aiding. A separate heater coil is not needed, inasmuch as suflicient resistive heating will be developed in the induction coils 90 and 92 to maintain the lamp 12 at the proper operating temperature.

' In operation, when the oscillator circuit is energized, a high frequency axial electric field is produced across the tank capacitor 72 and appears in the space between the coils 90 and 92 and directed along the axis of the lamp 12. In addition, the two coils 90 and 92 produce an axial magnetic field which induces a circumferential electric field about the axis of the lamp 12 to enhance the light emission from the lamp. Although the condensing member 46 is positioned within the induction coil 90, it is so much smaller in diameter than the coil that little, if any, heating current will be induced therein. As a result, the requirement that the condensing member 46 be maintained cooler than the envelope 40 walls is fulfilled. I

In the foregoing embodiments it has been assumed that the ambient temperature surrounding the lamp 12 is lower than the temperature within the envelope 40 so that the condensing member 46 may transfer heat to the cooler ambient. However, in instances where the ambient temperature is actually higher than the temperature within the envelope 40, some additional means must be provided to extract heat from the condensing member =46. One arrangement for accomplishing this is shown in Figs. 4 and 5. In this arrangement, the heat sink 66, preferably without fins 68, is surrounded by a tubular thermoelectric cold junction 94. The cold junction 94 may comprise two half cylinders 96 and 98 of two dissimilar metals, such as antimony and bismuth respectively, joined together to form a tube around the heat sink 66. A pair of conductors 102 and 104 are connected to the half cylinders 96 and 98 for supplying current to the junction 94 from a direct current source 106. As is well known, with the direct current source 106 connected to send current in the direction of the arrow from the antimony half cylinder 96 to the bismuth half cylinder 98, the junction 94 will be cooled. The cooling eflfect of the junction 94 may be used to lower the temperature of the condensing member 46 below that of the lamp envelope 40. This effect is known in the art as Peltier cooling.

Other lamp structures and arrangements of electromagnetic field exciting means may be used to concentrate the light emission within a relatively small region of the lamp so as to closely simulate a point source of light. In the arrangement shown in Fig. 6, a different lamp 108 is substituted for the lamp 12 shown in the arrangement of Fig. 3. The envelope 110 of the lamp 108 is provided with an annular bulge 112 approximately midway between the ends of theenvelope 110. Two induction coils 114 and 116 are mounted on the envelope 110, one on each side of the bulge 112 with the coils 114 and 116 being connected in series opposing rather than in series aiding, as in the embodiment of Fig. 3.

When the coils 114 and 116 are energized, two different components of magnetic field are produced by each of the coils. One field component produced by each of the coils, indicated by the arrows 118 and 120, is directed axially of the coils. The two axial components 118 and 120 are directed opposite each other and therefore cancel one another. Another field component produced by each of the coils, indicated by the arrows 122 and 124, is directed radially in the region of the bulge 112. The radial components 122 and 124 have the same direction and therefore add to one another to produce a resultant radial magnetic field in the region of the bulge 112. The radial magnetic field induces within the bulge 112 a high frequency circumferential electric field that is directed at right angles to the radial magnetic field and that has suflicient strength to ionize the gas and vapor within the lamp-108. Since the electric field is concentrated in the region of the bulge 112 there will be a concentration of ionization and hence light emission from that region. Thus both unobstructed light and a smaller light emitting area result from this arrangement.

It is now apparent that the novel and improved arrangements of the invention prove useful in reducing light fluctuations in, and improving the light output efliciency of, electrodeless vapor discharge lamps.

What is claimed is:

1. A light source comprising, in combination: a vapor discharge lamp including an envelope, a vaporizable substance within said envelope the vapors of which are adapted to emit light when subjected to an electromagnetic field, a vapor condensing member disposed Within said envelope and positioned remotely from the transmission path of light emitted from the light source and constructed of a material having an aflinity for said vaporizable substance, heat dissipating means disposed exteriorly of said envelope and thermally coupled to said condensing member to maintain the temperature of said condensing member below that of said envelope during light emission periods of the light source; and electro magnetic field producing means positioned along a portion of said lamp; said condensing member being sufficiently spaced from said electromagnetic field producing means as to be inappreciably within the influence of the electromagnetic field produced by said field producing means.

2. A light source comprising, in combination: a vapor discharge lamp including a cylindrical envelope, a vaporizable substance within said envelope the vapors of which are adapted to emit light when subjected to an electromagnetic field, an elongated vapor condensing member disposed Within and axially with said envelope and constructed of a material having an affinity for said vaporizable substance, and a heat sink disposed exteriorly of said envelope and thermally coupled to said condensing member to maintain the temperature of said condensing member below that of said envelope during light emission periods of the light source; and at least one induction coil encircling a portion of said lamp for producing an electromagnetic exciting field for said lamp; said condensing member being sufliciently spaced from said induction coil as to be inappreciably within the infiuence of the electromagnetic field produced by said induction coil.

3. A light source comprising, in combination: a parabolic light reflector having a predetermined focal point; a vapor discharge lamp including an elongated envelope axially aligned with said reflector, with a central portion of said envelope symmetrically positioned at said focal point, a vaporizable substance within said envelope the vapors of which are adapted to emit light when subjected to an electromagnetic field, a vapor condensing member disposed within said envelope in a region separate from said central portion and constructed of a material having an affinity for said vaporizable substance, heat dissipating means disposed exterionly of said envelope and thermally coupled to said condensing member to maintain the temperature of said condensing member below that of said envelope; and electromagnetic field producing means including elements positioned along said envelope on opposite sides of said central portion with said elements connected to produce substantially higher concentration of electromagnetic field in the region of said central envelope portion than in other regions of said envelope, whereby said condensing member is disposed in a region of relatively low field.

4. A light source according to claim 3, wherein said parabolic light reflector constitutes one of said electromagnetic field producing elements, and another one of said field producing elements is constituted by an induction coil connected to said reflector.

5. A light source according to claim 3, wherein said electromagnetic field producing means includes a pair of oppositely positioned induction coils.

6. A light source according to claim 5, wherein said induction coils are connected in series aiding.

7. A light source according to claim 5, wherein said induction coils are connected in series opposing to thereby create a radial electromagnetic field therebetween.

8. A light source according to claim 7, wherein the central portion of said lamp envelope comprises a bulbous annular portion of greater diameter than the remainder of said envelope whereby a greater portion of the radial field created by said opposing coils influences said vapors of said vaporizable substance.

9. A light source comprising, in combination: a parabolic light reflector having a predetermined focal point; a vapor discharge lamp including a cylindrical envelope axially aligned with said reflector, with a central portion of said envelope symmetrically positioned at said focal point, a vaporizable substance within said envelope the vapors of which are adapted to emit light when subjected to an electromagnetic field, a vapor condensing member disposed within said envelope in a region separate from said central portion and constructed of a material having an affinity for said vaporizable substance, and heat dissipating means disposed exteriorly of said envelope and thermally coupled to said condensing member to maintain the temperature of said condensing member below that of said envelope; and electromagnetic field producing means including elements positioned along said envelope on opposite sides of said central portion, at least one of said elements being an induction coil surrounding an end portion of said envelope, with said elements connected to produce a substantially higher concentration of electromagnetic field in the region of said central envelope portion than in other regions of said envelope, whereby said condensing member is disposed in a region of relatively low field.

10. A light source comprising, in combination: a vapor discharge lamp including an elongated envelope, a vaporizable substance within said envelope the vapors of which are adapted to emit light when subjected to an electromagnetic field; a light reflector including a portion encircling one end region of said envelope and having an axis along which said envelope is disposed; an electromagnetic field producing element encircling another end region of said envelope so as to expose a central region of said envelope; and electric circuit means connected to said reflector and said field producing element to establish an electromagnetic field therebetween, with a concentration of said field in said exposed central envelope region.

11. A light source comprising, in combination: a vapor discharge lamp including an elongated envelope, a vaporizable substance within said envelope the vapors of which are adapted to emit the light when subjected to an electromagnetic field, a vapor condensing member disposed within one end region of said envelope and having an aflinity for said vaporizable substance, heat dissipating means disposed exteriorly of said envelope and thermally coupled to said condensing member to maintain the temperature of said condensing member below that of said envelope; a light reflector including a neck portion encircling said one end region of said envelope and a reflecting portion joined to said neck portion and having a surface curved in directions away from said neck portion, said reflector having an axis along which said envelope and condensing member are disposed; said condensing member being substantially coextensive with said reflector neck portion; an electromagnetic field producing element encircling another end region of said envelope so as to expose a central region of said envelope; and electric circuit means connected to said reflector and said field producing element to establish an electromagnetic field therebetween, with a concentration of said field in said exposed central envelope region.

12. A light source comprising, in combination: a housing; a vapor discharge lamp mounted within said housing and including a cylindrical envelope, a vaporizable substance within said envelope the vapors of which are adapted to emit the light when subjected to an electromagnetic field, a rod-like vapor condensing member disposed axially within said envelope and made of a metal having an aflinity for said vaporizable substance, heat dissipating means disposed exteriorly of said envelope and thermally coupled to said condensing member to maintain the temperature of said condensing member below that of said envelope; a light reflector mounted within said housing and including a neck portion encircling one end region of said envelope and a reflecting portion joined to said neck portion and having a parabolic surface curved about a focal point spaced from said neck portion, said reflector having an axis along which said envelope and condensing member are disposed; said condensing member being substantially coextensive with said reflector neck portion; an electromagnetic field producing element encircling another end region of said envelope so as to expose a central region of said envelope, with the focal point of said reflector lying within said central region; and electric circuit means connected to said reflector and said field producing element to establish an electro-magnetic field therebetween, with a concentration of said field in said exposed central envelope region.

13. A light source according to claim 12, wherein said envelope and condensing members are concentrically cylindrical, with the diameter of said condensing member being of the order of one-fifth to one-tenth the outside diameter of said envelope.

14. A light source comprising, in combination: a vapor discharge lamp including an envelope, a vaporizable substance within said envelope the vapors of which are adapted to emit light when subjected to an electromagnetic field, a vapor condensing member disposed within said envelope and constructed of a metal having an affinity for said vaporizable substance, a heat dissipating member disposed exteriorly of said envelope and thermally coupled to said condensing member, a thermoelectric cold junction surrounding said heat dissipating member and thermally controllable to maintain the temperature of said condensing member below that of said envelope during light emission periods of the light source; and electromagnetic field producing means positioned along a portion of said lamp; said condensing member being sulficiently spaced from said electromagnetic field producing means as to be inappreciably within the influence of the electromagnetic field produced by said field producing means.

15. A light source comprising, in combination: a vapor discharge lamp including a cylindrical envelope, a vaporizable substance within said envelope the vapors of which are adapted to emit light when subjected to an electromagnetic field, a rod-like vapor condensing member disposed axially within said envelope and constructed of a metal having an aflinity for said vaporizable substance, a heat dissipating member disposed exteriorly of said envelope and thermally coupled to said condensing member to maintain the temperature of said condensing member below that of said envelope; and electromagnetic field producing means positioned along an exterior portion of said lamp; said condensing member having a substantially smaller diameter than that of said envelope so as to be inappreciably within the influence of the electromagnetic field produced by said field producing means.

16. A light source according to claim wherein said condensing member is mounted within one end portion of said envelope; and said electromagnetic field producing means includes an induction coil mounted on said one end portion and encircling said condensing member.

17. A light source according to claim 15 wherein said condensing member is mounted within one end portion of said envelope; and said electromagnetic field producing means includes an induction coil mounted within another end portion of said envelope and is spaced axially from said condensing member.

18. A light source comprising, in combination: a para-. bolic light reflector having a predetermined focal point;

a vapor discharge lamp including a substantially cylin J drical envelope axially aligned with said reflector, with a central portion of said envelope symmetrically posi tioned at said focal point; a vaporizable substance within said central portion of said envelope the vapors of which, when subjected to an electromagnetic field, emit light in a radial path from said central portion to said reflector; a vapor condensing member disposed within said field producing means including elements positioned along said envelope on opposite sides of said central portion, at least one of saidelements being an induction coil surrounding an end portion of said envelope adjacent to said central portion, with said elements coupled to produce a substantially higher concentration of the electromagnetic field in the region of said central portion of said envelope than in other regions of said envelope, whereby said condensing member is disposed in a region of relatively low field and is removed from the light path between said central portion of said envelope and said reflector.

19. A light source comprising, in combination: a light reflector having a predetermined focal point; a vapor discharge lamp including an envelope having a central portion symmetrically positioned at said focal point; a vaporizable substance within said central portion of said envelope, the vapors of which, when subjected to an electromagnetic field, emit light in a path from said central portion to said reflector; a vapor condensing member disposed within said envelope in a region separate from said central portion and constructed of a material having an afllnity for said vaporizable substances; heat dissipating means disposed exteriorally of said envelope and thermally coupled to said condensing member to maintain the temperature of said condensing member below that of said central portion, and electromagnetic field producing means to produce a substantially higher concentration of the electromagnetic field in the region of 7 said central portion of said envelope than in other 'j regions of said envelope, whereby said condensing mm 7 her is disposed in a region of relatively low field and is removed from the light path between said central portion of said envelope and said reflector.

References Cited in the file of this patent UNITED STATES PATENTS 1,874,478 Fayer Aug. 30, 1932 1,925,110 Parker Sept. 5, 1933" 2,027,519 Davis et al. Ian. 14, 1936 I 2,419,128 Evans ..---a------- Apr. 15, 1947 

